The present invention relates to the field of pyrotechnic gas generators. More precisely the invention relates to a pyrotechnic gas generator comprising a solid charge of plastic-bonded powder of the xe2x80x9cLovaxe2x80x9d type which can be employed especially in protective devices using an inflatable cushion.
Pyrotechnic gas generators intended to inflate protective cushions must supply a desired quantity of gas in a very short time, of the order of at most a few tens of milliseconds, without presenting any risks of explosion due to an untimely increase in the pressure within the generator.
Conventional pyrotechnic generators generally comprise a solid charge consisting either of a propellant powder based on nitrocellulose and nitroglycerine, also called a double-base powder, or of a propellant powder based on sodium azide.
Both these propellant compositions exhibit the advantage of being easily ignitable and of having high rates of combustion. Since double-base powders also have a rate of combustion which is not very dependent on pressure in the functioning range of motor vehicle safety, these compositions therefore do not present any risk of explosion of the generator, related to a runaway of the rate of combustion following an overpressure within the generator. Sodium azide-based compositions have rates of combustion which are more pressure-sensitive. Various safety measures have been proposed to avoid the risks of explosion of the generator with these compositions. In U.S. Pat. No. 4,066,415 it has thus been proposed to equip the generator with an auxiliary safety valve which opens when the pressure inside the combustion chamber exceeds a predetermined value, this auxiliary valve not being the standard exit orifice for the combustion gases.
In U.S. Pat. No. 3,897,962 it has also been proposed to equip the exit orifice for the combustion gases with a nozzle comprising prefragmentation lines so as to permit a widening of the gas exit section in the event of overpressure within the generator.
Finally, for generators with multiple and successive combustion chambers, patent EP-A-0 404 572 has proposed to equip the generator with nonreturn valves to prevent a chamber from being repressurized by another one.
However, conventional double-base compositions or those comprising sodium azide exhibit major disadvantages to a person skilled in the art. In fact, double-base compositions have a mediocre stability over time as a result of the phenomena of nitroglycerine migration, which are difficult to overcome and to control. Furthermore, sodium azide-based compositions produce large quantities of solid residues which must be filtered before the gases enter the inflatable cushion.
For these reasons people skilled in the art have been attempting for several years to abandon these conventional compositions, in order to employ compositions based on plastic-bonded powders which consist chiefly of an organic binder and of an energetic filler and which present the twin advantage of being chemically stable over time and of not producing solid residues when the energetic filler is properly chosen. Such compositions are also known by the English name of xe2x80x9cLovaxe2x80x9d powders because of their low vulnerability to mechanical or thermal shocks.
However, the use of plastic-bonded pyrotechnic charges presents a person skilled in the art with a double problem. On the one hand these compositions have rates of combustion which are sensitive to the surrounding pressure, and the generator must be equipped with safety devices against overpressures, but, on the other hand, these compositions are difficult to ignite and are very sensitive to any fall in pressure at the moment of ignition. The safety measures referred to above, all of which rely on the use of gas exit orifices which are calibrated from the outset for normal functioning during combustion, cannot therefore be applied with these new charges because they run the risk of resulting in their extinction when they begin to function.
To control the problem of the ignition of such charges, patent EP-A-0 570 347 has proposed a generator whose exit orifice is equipped with a valve that can close again in the event of a fall in the internal pressure during the ignition, so as to prevent the extinction of the charge. This solution is technically very tricky to control so as to be reliable within the functioning period of a few milliseconds that are required for motor vehicle safety; it is costly and, above all, it absolutely does not control the problem of risks of overpressure during combustion once the ignition is successful.
A person skilled in the art who attempts to employ plastic-bonded solid pyrotechnic charges does not therefore have available, at the present time, a generator which guarantees, within the essential time requirements demanded by motor vehicle safety, both a good ignition of the charge and the absence of risks related to internal overpressures during the combustion of the charge.
The objective of the present invention is precisely to propose such a generator. The invention therefore relates to a pyrotechnic gas generator intended especially for motor vehicle safety and comprising a hollow body provided with a gas exit opening, an ignition device and a pyrotechnic charge of xe2x80x9cLovaxe2x80x9d type, consisting chiefly of an organic binder and of an energetic filler, the rate of combustion of the said charge reaching a predetermined minimum value when the pressure to which it is subjected is higher than a value Po, characterized in that the said gas exit opening is closed by a device for discharging gases, comprising a component which has passing through it a cylindrical orifice closed by a cover which is ruptured when the internal pressure Pc of the generator is higher than the said pressure Po, the initial diameter of the said cylindrical orifice being calculated such that the instantaneous fall in pressure within the generator when the said cover opens should be lower than the difference Pcxe2x88x92Po, the said component being furthermore made of a material which can be eroded under the effect of the flow of the combustion gases of the said charge.
Thus, by virtue of the invention, the ignition of the plastic-bonded charge is ensured in a closed vessel which opens outwards, by bursting of the cover, only when the internal pressure of the generator has exceeded the threshold pressure Po which ensures a predetermined minimum rate of combustion. Furthermore the initial diameter of the cylindrical gas exit orifice is calculated such that after opening of the cover the internal pressure of the generator should not fall back below this threshold pressure. The generator according to the invention thus avoids the risks of extinction of the charge on ignition. Subsequently, in the course of functioning, the risks of overpressure within the generator are eliminated by the fact that the section of the orifice for discharging the combustion gases increases progressively owing to the erosive effect of the latter.
According to a first preferred embodiment of the invention the said erodable component is in the shape of a cylindrical ring which has a central cylindrical orifice. This erodable ring will be preferably also made of a resin chosen from the group consisting of polyamides, polyethers and polycarbonates. The said resin may be filled with glass fibres or carbon fibres.
According to a second preferred embodiment of the invention the said hollow body consists of a cylindrical casing one end of which consists of a substantially planar wall and the other end of which is closed by a closure ring supporting the ignition device. The said closure ring will advantageously be fixed by crimping in the open end of the said casing. Also advantageously the said gas exit opening is a central cylindrical opening carried by the substantially planar wall. In a particularly preferred alternative form the said cylindrical gas exit opening has, from the outside of the generator towards the inside of the latter, an outer part of large diameter, a middle part of small diameter and an inner part of intermediate diameter, in which is placed the cylindrical ring whose central cylindrical orifice must have an initial diameter smaller than that of the middle part of the gas exit opening. In this embodiment the device for discharging the gases consists simply of the opening of special geometry, of the erodable ring and of the cover described a little later. This ring will in general be adhesively bonded to the inside of the inner part of the cylindrical gas exit opening. It should be noted that in this embodiment the central orifice of the erodable ring can at most reach the same section as that of the middle part of the gas exit orifice. This embodiment thus permits the use of materials which are very easily erodable, since the erosion of the ring is limited by the structure of the gas exit opening.
Also advantageously, in this embodiment, the cover initially shutting off the gas exit orifice consists of a metal sheet fixed by adhesive bonding, by welding or by brazing against the internal surface of the said substantially planar wall. This metal sheet may be given the shape of a crimped or spun component which will be held in place in the crimping of the casing on the closure ring.
According to a third preferred embodiment of the invention the preferred plastic-bonded pyrotechnic compositions for forming the pyrotechnic charges of xe2x80x9cLovaxe2x80x9d type are either those based on oxygen-containing thermoplastic binder, on ammonium nitrate as oxidizing filler and of glycidyl azide polymer as plasticizer, and described in patent application FR-A-2 713 632, or those based on silicone binder and on a mixture of ammonium perchlorate and sodium nitrate, as described in patent application FR-A-2 728 562.
As a result of their structure and as a result of the use of plastic-bonded pyrotechnic charges, the generators according to the invention exhibit both a very high reliability of conservation over time and a very high reliability of functioning. Furthermore the generators according to the invention do not produce, or produce only a very small amount of, solid residues; they therefore do not require any complex filtering systems. Furthermore these generators exhibit a lower sensitivity during firings at elevated temperature, that is to say firings performed at +85xc2x0 C. The generators according to the invention can be employed directly as pyrotechnic generators intended to inflate protective cushions for the occupants of a motor vehicle, or indirectly as pyrotechnic components of more complex generators, for example so-called xe2x80x9chybridxe2x80x9d generators incorporating a reserve of gas under pressure.